A characteristic of ordinary perceptual hearing loss (sensorineural loss) is that the threshold of hearing is elevated, whereas the discomfort level is almost normal. This corresponds to a reduction in useful input dynamic range, which is also termed ‘recruitment’. The method for compensation of this deficit is to apply compression in the hearing aid. Compression is the most important signal processing used in modern hearing instruments. It addresses a number of issues such as:                Compensates the recruitment, which means ‘normalizing’ the dynamic range of the ear by matching the input dynamic range of the real-world to the restricted dynamic range of the client.        Adjusts different listening situations, both very soft and very loud, to be audible and yet never uncomfortably loud.        Enhances weak passages in the speech signal when in noisy situations. This can improve the speech intelligibility in noise.        
Further, multi-channel compression in separate frequency bands may reduce interaction between different signals at different frequencies.
The efficacy of the solution to all these issues depends on a number of compression parameters which must be carefully selected according to the main goal of the compression. Important parameters are compression kneepoint, compression ratio and time constants (attack & release). There are also a number of drawbacks to traditional compression:                Compression distortion—poor sound quality due to the squeezing of the dynamic range. I.e. the signal sounds ‘squeezed’ or ‘flat’. This is mostly the case for fast-acting compression, i.e. using short time constants for attack & release.        Pumping due to slow release from compression. This means that the softer sounds are suppressed for some time following a loud sound, while the gain is increased again (release). This is an effect of long release time constants.        
Poor protection against sudden loud sounds—this is an effect of slow time constants for attack.                Reduction of temporal contrast in the signal, i.e. the fast level variations—this may reduce speech intelligibility.        
In the case of multi-channel compression, the spectral contrast is also reduced which may reduce speech intelligibility. Furthermore the dynamic changes in timbre are perceived as unstable sound, and thus reduced sound quality.
A number of solutions to the disadvantages have been proposed and may patents written, e.g. Oticon's OGLE compressor by Neumann (WO 2003081947 A1), Ludvigsen (U.S. Pat. No. 6,628,795), and Salmi & Scheller (WO 9818294 A1). A typical solution is to combine fast and slow time constants and possibly make the level dependent, e.g. slow time constants for low levels and fast time constants for high levels. From EP1465456 a signal processing system, such as a hearing aid system is known, adapted to enhance binaural input signals. From US 2004/0190734 A1 a multi-channel signal processing system adapted to provide binaural compressing of tonal inputs is provided.